Semiconductor structure with a titanium aluminum nitride layer and method for fabricating same

ABSTRACT

A semiconductor device having a contact layer and a diffusion barrier layer is fabricated by preparing a semiconductor substrate, forming a layer of titanium/aluminum alloy on the surface of the substrate, and then heating the resultant structure in a nitrogen ambient to form a contact layer of titanium silicide interposed between the substrate and a diffusion barrier layer consisting of titanium/aluminum/nitride.

FIELD OF THE INVENTION

[0001] The present invention relates in general to fabricatingsemiconductor devices, and particularly to a new method for fabricatinga diffusion barrier layer consisting of a titanium/aluminum/nitridealloy.

BACKGROUND OF THE INVENTION

[0002] Integrated circuits, commonly referred to as semiconductordevices, are fabricated from materials with varying electricalproperties. The materials fall into one of three general groupings:electrical conductors, such as aluminum; electrical semiconductors, suchas silicon; and electrical nonconductors, such as silicon dioxide. Onestep in manufacturing semiconductor devices is metallization, whichrefers to the formation of metal films used for interconnections, ohmiccontacts, and rectifying metal/semiconductor contacts. Aluminum andtungsten are commonly used for metallization due to their lowresistivity. In state-of-the art semiconductor device architecture it iscommon for the metallization process to result in interfaces betweensilicon and the metal. It is well known that even at room temperaturethe metal and silicon will inter-diffuse over time. The dissolution ofsilicon into the metal also occurs when the device is annealed becausethe elements are subject to an elevated temperature, causing a rapidinterdiffusion of the metal and the silicon. Interdiffusion of the twomaterials changes the semiconductive properties of the silicon andcauses defective devices.

[0003] In order to control the interdiffusion it is common practice tomanufacture a diffusion barrier at metal/silicon interfaces insemiconductor devices. One method used to minimize the dissolution ofthe silicon is to add silicon to the metal by cosputtering until theamount of silicon contained by the metal satisfies the solubilityrequirement. Another method of satisfying the silicon requirements ofthe metal film is to deposit the film on a layer of doped polysilicon. Athird method is to introduce a barrier metal between the metal film andthe silicon substrate. To be effective the barrier metal must form a lowcontact resistance with silicon, it must not react with silicon, and itsdeposition and formation must be compatible with the overall process. Athin film of titanium nitride or titanium tungsten is commonly used inthis method and provides an adequate barrier. These and otherconventional diffusion barriers, while generally effective at roomtemperature, tend to fail at more elevated temperatures. Many of thepreferred semiconductor fabrication processes, such as deposition,reflow, and annealing, require elevated temperatures. Conventionaldiffusion barriers can therefore limit the range of processes availablefor fabricating a semiconductor device.

[0004] More recently, it has been found that an alloy material composedof titanium, aluminum, and nitrogen is a promising material for barrierapplications in semiconductor manufacturing. Titanium/aluminum/nitridehas a lower resistivity than titanium nitride and is more compatiblewith high temperature processing. This material is described in U.S.Pat. No. 5,231,306, granted Jul. 27, 1993 to Meikle et al., entitled“TITANIUM/ALUMINUM/NITROGEN MATERIAL FOR SEMICONDUCTOR DEVICES”, andassigned to the same assignee, the details of which are incorporated byreference. Meikle teaches a method of manufacturingtitanium/aluminum/nitride by cosputtering titanium and aluminum orsputtering a titanium/aluminum alloy in a nitrogen or nitrogen/argonambient. Titanium/aluminum/nitride is more effective than conventionalbarriers, especially at elevated temperatures. The reactive sputterprocess is, however, slower and more expensive than other conventionaldeposition techniques. With the ever-growing demand for semiconductordevices and the need to minimize manufacturing costs there is a need tobe able to manufacture a titanium/aluminum/nitride diffusion barrierusing conventional deposition techniques.

[0005] The advantages of building smaller semiconductor devicescontaining more circuitry are well known: electronic equipment becomesless bulky, reducing the number of solder or plug connections improvesreliability, assembly and packaging costs are minimized, and circuitperformance is improved, in particular higher clock speeds. In order tomanufacture smaller devices with more circuits, each of the individualcircuit elements are smaller. This creates a need for a diffusionbarrier which is thinner while retaining its effectiveness at elevatedtemperatures. It also creates a need for materials with low resistivityin order to minimize the power requirements.

[0006] Silicides make up one class of materials which have lowresistivity and form a very stable interface with common substratematerials. Titanium silicide in particular is a silicide which has beenshown to provide very low resistivity. Silicides such as titaniumsilicide are expected to play an increasingly important role in futuremetallization. Current processes known in the art for manufacturingtitanium silicide include depositing the titanium on polysilicon (orsilicon) and sintering the structure to form a suicide, by co-depositingthe titanium and silicon by simultaneous sputtering or evaporation, orthrough chemical vapor deposition.

[0007] The present invention describes a new technique of formingtitanium/aluminum/nitride without having to resort to the slower andmore expensive reactive sputter process. Furthermore, the new method hasthe added benefit of allowing simultaneous formation of titaniumsilicide which provides low contact resistance when the material issputtered onto a silicon substrate. As a result the number ofmanufacturing steps is decreased.

SUMMARY OF THE INVENTION

[0008] The present invention provides a titanium/aluminum/nitridediffusion barrier for preventing interdiffusion at a metal/siliconinterface within a semi-conductor device. A titanium silicide layerbetween the diffusion barrier and the substrate is also created at thesame time the diffusion barrier is manufactured.

[0009] According to one aspect of the method of the present invention, asemiconductor device is provided, the device comprising a substrate, acontact layer, and a diffusion barrier layer. The contact layer isinterposed between the substrate and the barrier layer. The substrate issilicon, but in other embodiments may employ any other form of siliconor silicon compound or any other doping of silicon that is known in theart. The contact layer consists of titanium silicide, and the diffusionbarrier layer is comprised of a titanium, aluminum, and nitrogen alloy.In one embodiment the percentage of materials in the barrier layer are:0 to 25% aluminum, 25 to 55% titanium, and 40 to 55% nitrogen. Thethickness of the diffusion barrier is approximately 20% of theas-deposited film thickness. The thickness of the contact layer isapproximately 150 to 250% of the as-deposited film thickness.

[0010] In accordance with another aspect of the invention, a process offabricating a semiconductor device is provided, comprising the steps ofpreparing a semiconductor substrate and then forming a layer oftitanium/aluminum alloy over the semiconductor substrate. The alloy isnitridized by annealing in a nitrogen-containing gas ambient. Accordingto another aspect of the method of the present invention, thenitridizing step comprises furnace annealing in a nitrogen ambient.According to yet another aspect of the method of the present invention,the nitridizing step comprises rapid thermal annealing in anitrogen-containing gas ambient. In one embodiment the rapid thermalanneal time is between 0.1 and 2 minutes. In another embodiment theanneal temperature is between 500 and 900° C., and in yet anotherembodiment the temperature is maintained at 700° C. The nitridizing stepproduces a titanium silicide layer interposed between the substrate anda titanium/aluminum/nitride diffusion barrier layer.

[0011] The invention provides a method of manufacturing a diffusionbarrier which provides better protection against diffusion thanconventional diffusion barriers. The invention provides the addedadvantage of creating a low resistivity contact layer with no additionalmanufacturing steps.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1A is a cross section of a portion of a semiconductor deviceshowing a layer of titanium/aluminum alloy deposited on a siliconsubstrate before the device has been annealed, according to oneembodiment of the present invention.

[0013]FIG. 1B is a cross section of a portion of a semiconductor deviceshowing a diffusion layer and a contact layer deposited on a siliconsubstrate after the device has been annealed, according to oneembodiment of the present invention.

[0014]FIG. 2 is a graph showing auger electron spectroscopy data of aTi/Al alloy after high temperature rapid thermal annealing.

[0015]FIG. 3 is a table showing contact resistance data from threeseparate one hundred contact strings of p+ contacts.

DESCRIPTION OF THE EMBODIMENTS

[0016] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined by the appendedclaims.

[0017] The steps of the present invention are represented in FIGS. 1Aand 1B. The shown semiconductor device embodying the invention iscomprised of a substrate of lightly P-doped single crystal silicon 110.It is to be understood that the present invention also applies to anyother form of silicon or silicon compound or any other doping of siliconthat is known in the art. FIG. 1A shows a layer of titanium/aluminumalloy 120 deposited on the silicon substrate 110. In this embodiment thealloy layer 120 is about 500 angstroms thick. The semiconductor device100 is rapid thermal annealed at 500° C. for five minutes in a nitrogenambient. In another embodiment the temperature is between 500 and 900°C. In yet another embodiment the temperature is maintained at 700° C.and the anneal time is between 0.5 and 3 minutes. Furnace annealing in anitrogen ambient may be used in an alternate embodiment. FIG. 1B showsthe results. Annealing causes the aluminum within the alloy 120 to moveto the surface and form a diffusion barrier layer 124 comprised oftitanium, aluminum, and nitrogen. Interposed between the barrier layer124 and silicon substrate 110 is a contact layer 122 comprised oftitanium silicide. The thickness of the diffusion barrier isapproximately 20% of the as-deposited film thickness, and the thicknessof the contact layer is 150 to 250% of the as-deposited film thickness.In the shown embodiment the diffusion barrier layer 124 is about 100angstroms thick and the contact layer 122 is between 500 and 600angstroms thick. The thickness of any actual diffusion layer 124 orcontact layer 122 may vary along the horizontal axis due toinconsistencies in the substrate topology or the deposition process.

[0018] With the new process the improved diffusion barrier materialtitanium/aluminum/nitride is fabricated without having to use a lowthroughput reactive sputter process. In addition, the barrier andcontact layers are formed in the same annealing step, thereby reducingthe overall number of processing steps. Compared to the conventionalpractice of annealing titanium to form a titanium silicide interface anda titanium nitride diffusion barrier, the process disclosed by theinvention provides a more effective barrier more efficiently. Thetitanium/aluminum/nitride is more effective because it has barrierproperties superior to those of other commonly used materials such astitanium nitride. For submicron contacts and vias bottom step coverageis minimal and any added protection greatly enhances the reliability ofthe circuits.

[0019]FIG. 2 shows auger electron spectroscopy (AES) data of 550angstroms of sputtered titanium-aluminum alloy after a high temperaturerapid thermal annealing step. Prior to the high temperature anneal stepthe titanium-aluminum layer contained approximately 80% titanium and 20%aluminum. The graph of FIG. 2 shows the relative concentrations in thecontact layer (from approximately 100 to 1100 angstroms in depth) to beapproximately: silicon 40 to 60%; titanium 20 to 30%; nitrogen 20 to30%; aluminum less than 5%. The graph shows the relative concentrationsof the materials in the diffusion layer (from approximately 0 to 100angstroms in depth) to be approximately: titanium 25 to 55%; nitrogen 40to 55%; aluminum 0 to 25%.

[0020]FIG. 3 shows contact resistance data from three separate onehundred contact strings of p+ contacts. “Control” refers to a depositionof 600 angstroms of titanium using a 1:1 ration aspect collimator.“Experimental” refers to a deposition of 600 angstroms oftitanium-aluminum alloy using a non-collimated dual source module. Inboth cases deposition was followed by a nitrogen ambient rapid thermalannealing step. The data clearly show that low contact resistance can beobtained simultaneous to titanium/aluminum/nitride formation using thenew process. The dual source used for the experimental test has poorcontact step coverage and it is expected that contact resistanceequivalent to control can be obtained if collimated deposition or somealternate high step coverage deposition method is used.

[0021] It is to be understood that the above description is intended tobe illustrative. and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A semiconductor device comprising: a contactlayer comprising silicide; and a diffusion barrier layer adjacent to thecontact layer, the diffusion barrier layer comprising an alloy ofaluminum, nitride and a refractory metal.
 2. The semiconductor device ofclaim 1 wherein the refractory metal is titanium and the contact layeris titanium silicide.
 3. The semiconductor device of claim 1 wherein therefractory metal is tungsten and the contact layer is tungsten silicide.4. The semiconductor device of claim 1 wherein the refractory metal istantalum and the contact layer is tantalum silicide.
 5. Thesemiconductor device of claim 1 further comprising a semiconductorsubstrate, wherein the contact layer is disposed over the semiconductorsubstrate.
 6. The semiconductor device of claim 1 wherein the diffusionbarrier layer contains 0 to 25% aluminum.
 7. The semiconductor device ofclaim 1 wherein the diffusion barrier layer contains 25 to 55%refractory metal.
 8. The semiconductor device of claim 1 wherein thediffusion barrier layer contains 40 to 55% nitrogen.
 9. Thesemiconductor device of claim 1 wherein the diffusion barrier layer isapproximately 20% of the deposited film thickness.
 10. The semiconductordevice of claim 1 wherein the contact layer is approximately 150 to 250%of the deposited film thickness.
 11. A process of fabricating asemiconductor device comprising the steps of: forming a layer comprisingan alloy of aluminum and a refractory metal; and nitridizing the alloy.12. The process of claim 11 wherein the alloy comprises titanium andaluminum and the nitridizing step creates a titanium/aluminum/nitridelayer.
 13. The process of claim 11 wherein the alloy comprises tungstenand aluminum and the nitridizing step creates atungsten/aluminum/nitride layer.
 14. The process of claim 11 wherein thealloy comprises tantalum and aluminum and the nitridizing step creates atantalum/aluminum/nitride layer.
 15. The process of claim 11 wherein thenitridizing step comprises furnace annealing in a nitrogen-containinggas ambient.
 16. The process of claim 11 wherein the nitridizing stepcomprises rapid thermal annealing in a nitrogen-containing gas ambient.17. The process of claim 11 wherein the nitridizing step comprisesmaintaining the annealing temperature between 500 and 900° C.
 18. Theprocess of claim 11 wherein the nitridizing step comprises maintainingthe annealing temperature at 700° C.
 19. The process of claim 11 whereinthe nitridizing step comprises annealing the semiconductor device for aperiod between 0.1 and 3 minutes.
 20. The process of claim 11 furthercomprising preparing a semiconductor substrate and forming the alloyover the semiconductor substrate.
 21. The process of claim 20 whereinthe semiconductor substrate is silicon and wherein the nitridizing stepfurther comprises forming a silicide contact layer interposed betweenthe semiconductor substrate and the diffusion barrier layer resultingfrom nitridizing the alloy.
 22. The process of claim 20 wherein thesemiconductor substrate is polysilicon and wherein the nitridizing stepfurther comprises forming a silicide contact layer interposed betweenthe semiconductor substrate and the diffusion barrier layer resultingfrom nitridizing the alloy.